Especially for landing systems, availability and continuity of service (C.O.S.) are very important safety features. Therefore, the requirements for C.O.S. of the ground equipment are internationally standardized. The most stringent requirements are imposed on those systems which are intended to be used under zero-visibility weather conditions. One known method to comply with these requirements is the use of redundant equipment, e.g. dualized equipment. If the equipment currently in operation fails, the other equipment takes over and thus continues service.
Landing systems normally consist of three components: one component to derive the horizontal or azimuth angle, one component to derive the vertical or elevation angle, and one component to derive range or distance information. These three components provide the three coordinates necessary to determine the position of an aircraft. The international standards and recommended practices for ILS (Instrument Landing System) were published long ago and those for MLS (Mircowave Landing System) recently.
For a brief overview of MLS, reference is made to the MICROWAVE JOURNAL, Vol. 24, No. 5, May 1981, pages 113 to 120.
A method of monitoring a landing system ground station and providing uninterrupted operation by switching to the redundant equipment is dealt with in the European patent EP-A-0 200 598. A widely used structure is described, comprising two transmitters which are permanently monitored by two monitor units. When, for example, the transmitter in operation produces a signal which is out of tolerance, its monitor unit, through a control logic device, causes the redundant transmitter to be switched from a dummy load to the antenna. Such a structure is perfectly symmetrical, and can be used for MLS as well as for ILS. However, this conventional monitoring concept has the disadvantage of not being able to check the overall operation of the redundant transmitter, because only the signal of the operational transmitter can be monitored. With ILS, having a passive antenna, this drawback is of minor relevance. But for MLS and its electronic antenna, this concept may be unsatisfactory.
The solution disclosed in the above European patent application provides a method of monitoring a station of a landing aid system of the MLS type, said station transmitting MLS information grouped together in identical cycles, or transmission periods and comprising: an electronic antenna, two MLS information transmission equipments, switching means providing connection of said transmission equipment to said antenna, means for monitoring the operation of the station, including means for picking up the signal transmitted by said antenna; said method comprising: switching the two transmission equipments alternately to said antenna to thereby cause the transmission of MLS information signals alternately from each of the two transmission equipments to the antenna, testing signals with said monitoring means, said testing signals being supplied by said pick-up means, and switching said transmission equipment when an error is detected.